Chemical mechanical planarization, also known as chemical mechanical polishing or CMP, is a technique used to planarize the top surface of an in-process semiconductor wafer or other substrates in preparation of subsequent steps or for selectively removing material according to its position. The technique employs a slurry that can have corrosive and abrasive properties in conjunction with a polishing pad.
While many existing CMP pads are non-porous, porous polishing pads generally provide improved slurry transport and localized slurry contact.
One technique for making high density foam polishing pads includes agitating a liquid polymer resin at a controlled temperature and pressure, using a surfactant, to produce a stable froth. The resin froth can be metered under pressure to a mix head where it is typically combined with a desired amount of curative before being injected or poured into a mold.
Other techniques for introducing porosity into pad materials include incorporating beads or hollow polymeric microspheres into the material. In some instances, a polymeric matrix used to manufacture the pad has been combined with polymeric microelements that soften or dissolve upon contact with a polishing slurry.
Many existing CMP pads have pore size limitations imposed by the technique used to create the microstructure. Gas frothing, for instance, can produce wider pore size distributions, larger than 30 microns (μm), whereas microspheres-filled pads often have pore sizes greater than 20-30 μm, depending on the size of the microspheres.
Generally, CMP is a dynamic process involving cyclic motion of both the polishing pad and the workpiece. During the polishing cycle, energy is transmitted to the pad. A portion of this energy is dissipated inside the pad as heat, and the remaining portion is stored in the pad and subsequently released as elastic energy during the polishing cycle. The latter is believed to contribute to the phenomenon of dishing of metal features and oxide erosion.
One attempt to describe damping effects quantitatively has used a parameter named Energy Loss Factor (KEL). KEL is defined as the energy per unit volume lost in each deformation cycle. Generally, the higher the value of KEL for a pad, the lower the elastic rebound and the lower the observed dishing.
To increase the KEL value, the pad can be made softer. However, this approach tends to also reduce the stiffness of the pad. The reduced stiffness results in decreased planarization efficiency and increases dishing due to conformation of the pad around the device corner.
Another approach for increasing the KEL value of the pad is to alter its physical composition in such a way that KEL is increased without reducing stiffness. This can be achieved by altering the composition of the hard segments (or phases) and the soft segments (or phases) in the pad and/or the ratio of the hard to soft segments (or phases) in the pad.